1. Field of the Invention
The present invention relates to a nanophosphor which may be used as a wavelength conversion part of a solar cell, a fluorescent contrast agent, and a light emitting part of a display device, and a synthesis method thereof, and particularly, to the synthesis of a fluoride-based nanophosphor having a particle size of 2 nm to 60 nm and a tetragonal structure, which is excited by ultraviolet light to emit strong green light and emits visible light that can be controlled over the colors such as green, yellowish green, yellow, and orange colors only by adjusting the amount of a doping agent.
2. Background of the Invention
Nanophosphors have a structure in which a lanthanide element is doped into an oxide, fluoride, sulfide or nitride-based host material with a size of 100 nm or less. A nanophosphor doped with trivalent lanthanide ions except for Ce3+ ion exhibits an inherent luminescent color depending on the doped lanthanide element, regardless of a type of host material. This is because the light emission of the nanophosphor is generated by 4f-4f transition due to the transition of 4f electrons of the trivalent lanthanide ions to dope the host material. Therefore, there is an advantage in that a desired light emission wavelength may be maintained even though sizes of particles are differently adjusted, if necessary.
Since these nanophosphors, even though being doped with the same lanthanide element, show characteristics which are different in intensity of light emission according to the type of host material to be doped, an appropriate host material needs to be selected in order to obtain strong light emission.
Further, since these nanophosphors exhibit fixed light emission colors, such as red or green, according to the element to be doped, there is a disadvantage in that it is difficult to obtain a desired light emission color, except for several fixed colors. To overcome this problem, when two or more types of phosphors, which exhibit different light emission colors, are mixed, various colors may be implemented, but there is a disadvantage in that excitation light sources with different wavelengths need to be used. This is because each element has an inherent absorption wavelength range in the case of light emission via 4f-4f transition, and accordingly, the mixed phosphors do not all emit light under a single excitation wavelength. To solve this problem, Wang, et al., (Nanotechnology vol, 18. 025701 (2007)), could obtain light emission of green, red and bluish green colors, and the like using a single wavelength around 250 nm by emitting light through a process of coating Ce on NaGdF4 particles doped with each of Tb, Eu, Sm and Dy to absorb ultraviolet light around 250 nm and transferring the absorbed in energy to each lanthanide element. However, this case, only a green light emission may be obtained when Ce and Tb are co-doped, only a red light emission may be obtained when Ce and Eu are co-doped, only the red light emission may also be obtained when Ce and Sm are co-doped, and a bluish green light emission may be obtained when Ce and Dy are co-doped. That is, only an inherent light emission color emitted by each of Tb, Eu, Sm and Dy could be implemented. Further, the amount of the co-doping agent Ce is so low that the absorption efficiency of excitation light is low and accordingly, there is a disadvantage in that it is difficult to obtain strong light emission. In addition, when various light emission colors are implemented through energy transfer, the larger the amount of doping agent to which energy is transferred is, the weaker the light emission of the doping agent which transfers energy becomes, and accordingly, the stronger light emission of the doping agent which transfers energy is, the more advantageous it is.
For light emission of a nanophosphor, nanophosphor particles, which use light in an infrared wavelength range as excitation light, have been known in the related art. However, in this case, light emission occurs in an upconversion mechanism in which an infrared light with a wavelength of about 980 nm is converted into visible light, and thus the intensity of a light source must be strong. Accordingly, it is impossible to observe light emission using a common lamp, but laser needs to be used as a light source. Furthermore, it has been reported that upconversion nanophosphors generally have an efficiency less than 1%, whereas downconversion nanophosphors have an efficiency of 30 to 40% to 70% in some cases.
In addition, since a single-junction amorphous silicon solar cell has a very low efficiency when irradiated with light in an ultraviolet range, and a high efficiency when irradiated with light in a visible light range of 500 nm or more, the efficiency may be enhanced by a method of disposing a wavelength conversion material which may convert ultraviolet light into a visible light range on the front surface portion of a silicon solar cell. At this time, when the size of a wavelength conversion material disposed on the front surface portion of the solar cell, that is, the size of a phosphor is large, incident light is scattered, thereby leading to deterioration in efficiency of the solar cell. Therefore, a nanophosphor having a small phosphor needs to be used in order to minimize the scattering of incident light. In general, since light emission from the nanophosphor is weaker than that from a microphosphor powder, a nanophosphor having a very strong light emission should be applied in order to enhance the efficiency of the solar cell using a wavelength conversion from ultraviolet light to visible light.
Furthermore, when light emission of individual nanophosphors as in a bioimaging contrast agent is used, various types of nanophosphors may not be mixed and used in some cases. In general, organic dyes have been widely used as bioimaging contrast agents. The organic dyes have characteristics of exhibiting various light emission colors and showing high light emission intensity according to types. However, due to extremely low photostability, there is a disadvantage in that a slight increase in an exposure time to excitation light may lower light emission intensity drastically. To overcome the problem, attempts have been recently made to apply inorganic light emission materials, such as quantum dots as bioimaging contrast agents, but the quantum dots cause blinking of light emission, and are difficult to be applied when containing a heavy metal, such as Cd, as in the case of CdSe.
Therefore, there is urgent need for developing a new material which facilitates tuning of a light emission color, and exhibits high stability without blinking of light emission. When a nanophosphor which may emit light in various wavelength bands under one excitation wavelength is implemented, the nanophosphor may be applied as a bio imaging contrast agent capable of differentiating various materials, and also a display device with ultrahigh image quality capable of emitting light with various colors may be implemented.
Throughout the present specification, a plurality of papers and patent documents are referenced, and citations thereof are indicated. The disclosure of each of the cited papers and patent documents is incorporated herein by reference in its entirety to describe the level of the technical field to which the present invention pertains and the content of the present invention more apparently.